The present invention relates to a radial ply tire for heavy duty use suitable for aircraft tires which is improved in bead durability, and more particularly to an improved bead structure.
The heavy duty radial tires for aircraft are used under heavy load and high speed, and therefore, the tires should essentially resist repeated large deformations due to heavy loads and high speed in order to achieve safety during aircraft takeoff and landing. Due to the recent enlargement in aircraft size and acceleration of the flight speed, such tires must be designed so as to endure not only an increased speed and load at takeoff and landing but also to endure the repeated large radial deflection when loaded, which can be, for example, 28 to 35%.
Today the widely used tires for aircraft have a, cross-ply structure. In such tires, however, the stiffness at the tread is less and the tire weight is heavy and thus neither preferable wear resistance or desirable cooling properties can be obtained. Considering the rapid enhancement of performances in large-sized jet planes, there is a limit in the amount of use which can be made of the cross-ply structured tires.
Accordingly, there is a tendency to employ radial ply tires having a belt layer composed of high modulus belt cords arranged radially outside the carcass at a small angle with regard to the tire equator.
By employing such radial ply structure for heavy duty tires such as aircraft tires, the tire life has been extended. However, it has been determined that the durability at the bead portion of the tire becomes relatively low in comparison with that of the entire tire.
It is therefore a general object of the present invention to provide a heavy duty radial ply tire which is improved in bead durability.
In radial ply tires, as a characteristic of the radial structure, a strong and rigid belt is disposed in the tread portion, and accordingly, the deformation thereof is small.
On the other hand, in aircraft tires, the deflection or deformation when loaded is significantly large because of the action of heavier loads in comparison with tires in other fields, for example, those for passenger cars.
Accordingly, the sidewall portions and the bead portions of the tire become subjected to severe deformation due to the increase in the load as compared with radial tires in other fields and furthermore the aircraft tires of a bias structure.
Above all, since a bead apex of a comparatively large hardness rubber, which develops heat largely by deformation, is disposed in each bead region, the temperature of the bead apex rises due to the heat generated by the repeated deformation during the operations of takeoff and landing.
Moreover, a large number of reinforcement layers, such as carcass plies, bead reinforcing plies, etc. are placed in the bead portions, and accordingly the thickness thereof is increased. Therefore, when the bead portion is subjected to bending deformation, a large shear strain is produced between the adjacent plies, which induces breakage of the tire bead portion.
It is therefore another object of the present invention to provide a heavy duty radial ply tire in which, in order to prevent the breakage of the tire bead portion, a reduction in both the heat generation and the shear strain is achieved by arranging the bead core in a specific position relative to the wheel rim and by preliminarily forming the bead apex in a shape similar to that when the tire is loaded.
On the other hand, it was known that the breakage of the bead often appeared near the radially outer edge of the rim flange. This is explained as follows: When the deflection in the radial direction of the tire while loaded is as large as 28 to 35% as stated above, as shown roughly in FIG. 7, a tensile force is generated in the axially inside portion of the carcass (e) or generally the main portion (j) of the carcass, and in contrast therewith, a compressive stress is applied to the outside portion thereof such as the carcass ply turnup portion (h) turned up around the bead core (f), and when the bead portion is sharply folded over the rim flange (g), the compressive stress concentrates to build up a large compressive stress at the carcass placed in the folded portion, and such stress is repeated during the tire rotation. Accordingly, the ply cords are fatigued and finally broken. Furthermore, the broken ends give rise to the local concentration of compressive stress. Such repeated compressive stress leads to the breakage of bead.
In the conventional tires, the bead apex is designed to be thicker and taller in order to lessen the elastic deformation of the bead portion, and the distance between the turnup portion (h) and the main portion (j) of the carcass (e) is accordingly increased. As the result, the above-mentioned repeated compressive stress is also increased.
The inventors recognized that in order to reduce the compressive stress at the carcass turnup portion (h), it is effective to bring the radially outer edge close to the neutral line U in the bending of the bead portion.
To explain further in detail, assuming the deformation of the bead is constant under the conditions of high pressure and heavy load, the strain .epsilon. of the carcass is expressed in the following equation: EQU .epsilon.=y/R (1)
where
R: radius of curvature of the carcass and PA1 y: distance from the neutral line U in bending to the outside of the turnup portion of the carcass. PA1 E: Young's modulus PA1 I: moment of inertia of area PA1 M: bending moment applied at the bead.
That is, the strain .epsilon. of the carcass becomes smaller as the distance y is shorter, if it is assumed that R is substantially constant.
On the other hand, the fact that the deformation is not uniform depending on the strength of the carcass (e) yields the following equation: EQU R=EI/M (2)
where
In addition, since the moment of inertia of area I and the thickness T of the bead have the relation EQU I.varies.T.sup.3 ( 3)
equations (1), (2) and (3) deduce the relation ##EQU1##
As a consequence, it was discovered that the strain .epsilon. can be minimized by causing the axially outside of the carcass turnup portion to approach the above-mentioned neutral line U, and contacting the carcass turnup portion (h) with the carcass main portion (j), and further reducing the quantity of rubber in the triangular sectional region between the turnup portion and main portion of the carcass by making the bead apex smaller.
It is therefore another object of the present invention to provide a heavy duty radial ply tire in which, in order to improve the durability of the bead portion by alleviating the fatigue of the carcass cords caused by the repeated deformations, the compressive stress on the turnup portion is reduced by defining the height of the bead apex and the position of the carcass turnup portion with relation to the bead core and the rim flange.